1. Field of the Invention
The invention relates to steering mechanisms as used with relatively small lawn tractors or grass mowing vehicles.
2. Description of the Related Art
Lawn tractors or mowing vehicles typically operate on ground surfaces such as grass or turf that are easily damaged or scuffed by the wheels of the vehicle Such scuffing can occur as the vehicle wheels skid laterally instead of rolling forwardly. When a vehicle having two steerable wheels executes a turn, one of the steered wheels may be skidding slightly laterally. A wheel may skid slightly when the axes of rotation of the vehicle's ground engaging wheels do not intersect at a common point. Such inaccuracies in the execution of a turn are generally caused by the vehicle's steering mechanism.
Many lawn vehicles provide pinion and sector steering mechanisms. A pinion gear is coupled for rotation with the steering column and engages an arcuate sector gear. As the steering column and sector gear rotate, the sector gear swings about a pivotal coupling with the vehicle. Tie rods are operatively coupled with the sector gear, and transfer the movement of the sector gear to the ground engaging steered wheels. However, many pinion and sector steering mechanisms are relatively inaccurate, and therefore can cause scuffing of the ground, particularly in tight cornering. These inaccuracies are caused in large part by the point of connection between the tie rods and the sector gear being shifted longitudinally as well as laterally as the sector gear rotates.
In an attempt to reduce the steering errors associated with pinion and sector mechanisms, sector gears having relatively large radii have been provided to reduce the longitudinal shifting of the tie rods as the sector rotates. However, increasing the size of the sector also increases the weight, complexity and cost of the mechanism.
Many automobiles provide rack and pinion steering mechanism, which generally provide better turning accuracy than sector-pinion mechanisms. A rack gear having a straight or non-curved toothed portion engages the pinion. Rack gears may be conceptualized as a sector gear having an infinite radius, such that the gear teeth are arranged in a straight line. Since the rack gear teeth are aligned in a straight line, the rack gear will shift laterally in response to rotation of the pinion gear and will have no component of motion in the longitudinal direction. Therefore, the point at which the tie rods are coupled to the rack gear will not shift longitudinally during a turn, and turning accuracy is generally enhanced.
However, the rack and pinion steering mechanisms used with automobiles are generally unfeasible for use with lawn vehicles. Prior art rack and pinion mechanisms used with automobiles typically have a large housing for enclosing the rack gear and pinion gear in a lubricated environment. Such housings have a relatively large number of parts, and are relatively heavy and costly. Automotive type housings are typically too large to be carried within the small compartments provided by lawn and garden tractors. Mechanisms are often provided within the automotive type housings for limiting the angle at which the steerable wheels can be turned. Maintenance is required to keep the contents of the housing lubricated. Bearing support mechanisms must be provided within the housing for accurately maintaining the rack gear in meshed engagement with the pinion gear. The small tolerances involved in the use of such bearing support mechanisms significantly increases the cost of the steering mechanism. The rack and pinion steering mechanisms used in automobiles are generally too costly, complicated, and heavy to be feasible for use with lawn vehicles.
In addition, automobile rack and pinion gear mechanisms typically provide tie rods that extend from the laterally outer edge portions of the rack gear to the steerable wheels. As the rack gear shifts laterally during execution of a turn, the tie rods shift to pivot the steerable wheels. Since the tie rods are connected to the outer edges of the rack gear, the automobile type rack and pinion mechanisms are relatively wide, and are therefore generally unsuitable for use with relatively narrow lawn and garden vehicles.
Many small lawn vehicles provide a front axle that pivots about a connection at the centerline of the vehicle. The front steerable wheels are pivotally carried at the outer edges of the pivotal axle. The axle pivots about the central connection with the vehicle to allow the front wheels to remain in contact with the ground when ground surface undulations or side slopes are encountered. As the axle and steerable wheels pivot about the center of the vehicle to ride over a side slope, the tie rods remain fixed to the rack gear, and therefore cause the steerable wheels to turn slightly as undulations are encountered. These steering inaccuracies, referred to as dynamic steering errors, may cause the turf beneath the tires to be undesirably scuffed. Dynamic steering errors can be reduced by coupling the tie rods to the steering mechanism near the point about which the axle and wheels pivot to ride over ground undulations. Therefore, for a vehicle with a front axle having a central pivot axis, the optimal location for coupling the tie rods to the rack gear would be near the center of the vehicle. However, automotive type rack and pinion mechanisms typically couple the tie rods to the outer edges of the rack gear, and therefore would create relatively large dynamic steering errors. The automotive type of rack and pinion mechanism is therefore generally unsuitable for use with small lawn vehicles having a front axles pivotable about a central point.
It may be desirable to provide a vehicle with a frame structure having generally enclosed box structures that serve to strengthen the frame structure. Such a frame structure might not provide access to the interiors of the enclosed boxes, and therefore may prevent certain vehicle components from being assembled or attached to the interior of the vehicle frame. Conventional steering mechanisms therefore may not be capable of being assembled or attached to frames having generally enclosed and inaccessible box structures.
Therefore, it would be desirable to provide a steering mechanism of the rack and pinion type that is comprised of few parts and is inexpensive to manufacture, and that is adapted for use with small lawn vehicles. It would be desirable for such a mechanism to be carried by the vehicle in such a way that no complex or costly housing structure is needed. It would also be desirable for the rack gear of such a mechanism to be secured in meshed relationship with the pinion gear in such a way that complicated and costly bearing support structures are not required. It would be desirable for such a mechanism to be adapted for use on a relatively narrow vehicle such as a lawn and garden vehicle. It would be desirable to provide such a steering apparatus with a mechanism for limiting the extent to which the steerable wheels can be turned, and that reduces or eliminates dynamic steering errors. It would also be desirable to provide a steering mechanism capable of being assembled on a vehicle having generally enclosed or inaccessible box structures formed by the vehicle frame.